The invention relates to brake disks for motor vehicles, in particular passenger cars and motorcycles, and to a steel alloy, as well as to a method for producing them.
Brake disks for passenger cars consist of a one- or two-part brake ring (solid or air-cooled brake disk), which is connected with a cup-shaped hub (FIG. 1). The brake disk is fastened on the axle by means of the cup, and at the same time is rigidly fastened to the wheel by means of a screw connection. Frictional contact takes place between the brake ring and the brake linings on both sides when actuating the brake by means of hydraulic pressure. The frictional heat generated in the course of this, independently of the brake pressure and the circumferential speed, is removed by way of the brake ring and the hub. Depending on the driving conditions, temperatures of up to 600 to 800xc2x0 C. can be reached on the surface of the brake disks of passenger cars. Under wet conditions a subsequent rapid cooling of the brake disk can take place. Stresses due to temperature changes are generated in the brake disk, in particular in the brake ring, through repeated sequential braking which, together with mechanical abrasion and corrosion, lead to damage of the material and limit the service life.
At present, brake disks for passenger cars are usually made of cast iron containing laminar graphite with approximately 2.5 to 4.0% of carbon, 1.8 to 2.5% of silicon, 0.3 to 1.0% of manganese, as well as trace amounts of other alloy elements. Advantages of the cast iron brake disks are the relatively low production costs and a heat conductivity which, in comparison with steel, is approximately twice as high and adds to high thermal load-carrying capacity along with good braking behavior. However, disadvantages of cast iron brake disks are their insufficient corrosion resistance, high susceptibility to wear, relatively high weight, as well as visible deficiencies because of corrosion.
The object of the invention is therefore based on making available a more corrosion-resistant brake disk for motor vehicles, and in the course of this to assure at least an equivalent braking behavior in comparison with cast iron disks for passenger cars, to achieve increased service life through greater wear resistance, to provide weight savings, and to achieve an improvement in visual appearance, which is desired in spoke-like wheels. In connection with brake disks for passenger cars, it is furthermore necessary to assure that no critical fracture formation, no deformations of the brake disk and no reduction of the braking effects occur in case of extreme braking with high thermal stresses.
In comparison with high-grade steel brake disks, which are usually employed today for motorcycles, but which are clearly less stressed in comparison with passenger car brakes, it was the object of the invention to find a material with higher corrosion resistance to aqueous hydrochloric acid, increased wear resistance and improved resistance to temperature changes (resistance to fractures under temperature change stresses).